We have prepared a number of clones of DNA segments from Drosophila and Necturus DNA. These can be highly labelled and used as probes to determine the number of times they are represented within the genome. Following this, we intend to determine their chromosomal location by in situ hybridization. Of particular interest will be the segments from necturus that are known to be present 10-100,000 times. We desire to know whether they are located in clusters, or whether they are distributed in a dispersed manner throughout the genome. A second approach deals with the annealing of these probes to 2-dimensional restriction "fingerprints" of these genomes. Our objective is to learn whether the genome is a random "montage" of inserted elements, or whether it is organized in clustered arrays of tandem sequences. BIBLIOGRAPHIC REFERENCES: D.H. Hamer and C.A. Thomas, Jr.: "Molecular Cloning of DNA Fragments Produced by Restriction Endonucleases Sal I and Bam I." Proc. Nat. Acad. Sci. USA 73, 1537 (1976). D.C. Gould, H.G. Callan, and C.A. Thomas, Jr.: "The Actions of Endonucleases R on Lampbrush Chromosomes." J. Cell. Sci., 21, 303 (1976).